The measurement of test surfaces by interferometry has generally been limited to planar or spherical surfaces, because test wavefronts matching these surfaces are easy to create. Two wavefronts are normally required--a test wavefront and a reference wavefront. Both start as identical wavefronts, but the test and reference wavefronts take different paths before recombining. Along the path of the test wavefront is the test surface. Any errors in the test surface alter the test wavefront with respect to the reference wavefront, and these differences are apparent in an interference pattern created by the recombined wavefronts.
The test wavefront can be either transmitted or reflected by the test surface to acquire relevant information for comparison. Non-optical test surfaces are measured by reflection, which is usually conducted at normal incidence. The test wavefront is shaped to approach a desired form of the test surface at normal incidence so that any errors in the test surface alter an otherwise retroreflected shape of the test wavefront. Planar surfaces are sometimes measured at grazing incidence in which the test surface folds a planar test waveform along a path independent of a corresponding planar reference wavefront.
Recently, I have been developing the use of diffractive optics for relatively shaping test wavefronts, particularly for measuring non-planar surfaces at grazing incidence. I have been preceded in this endeavor by Dr. Johannes Schwider, who in a 1974 East German Patent No. 106769 first proposed use of two identical diffraction gratings for measuring cylindrical surfaces at grazing incidence. The two gratings are aligned with an axis of the cylindrical surface. The first grating divides a planar primary wavefront into test and reference wavefronts and reshapes the test wavefront into an axiconic form for reflecting from the cylindrical test surface at a constant grazing angle. The reference wavefront passes directly to the second grating without change. The test and reference wavefronts are recombined by the second diffractive optic to produce an interference pattern.
For many years thereafter, little practical exploitation of Dr. Schwider's ideas was achieved. Various practical considerations were left unresolved including setups for actual test objects and further definitions of the gratings to measure other test surfaces. My recent efforts to advance this previously neglected area of technology are documented by a collection of U.S. patent applications, beginning with U.S. application Ser. No. 08/375,499 filed Jan. 19, 1995, which was replaced by U.S. application Ser. No. 08/483,737 filed Jun. 7, 1995, now U.S. Pat. No. 5,654,798. Another of my applications in which I am named as a co-inventor was filed on Jul. 31, 1995 as U.S. application Ser. No. 08/509,161, now U.S. Pat. No. 5,793,468. A co-assigned U.S. application Ser. No. 08/631,071 filed Apr. 12, 1996, now U.S. Pat. No. 5,719,676 further develops this technology. All of these applications are hereby incorporated by reference.